For a wide band amplifier it is necessary to maximize the gain-bandwidth product. It is possible to obtain the desired gain by using several amplification stages. However, the linearity and the stability of the amplification factor will be low in this case. Both parameters may be improved by employing negative feedback. However, negative feedback in multistage amplifiers has an inherent high frequency stability problem. Moreover, the time delay of multistage amplifiers is equal to the sum of the time delays for each stage.
The present invention aims to provide an amplifier with high gain and low bandwith degradation.
A prior art basic amplifier circuit design is shown in FIG. 1. The basic amplifier circuit shown is formed by a PNP input transistor Q1, connected in an emitter follower configuration and coupled to the base of an NPN transistor Q2, connected in a common emitter configuration. A current mirror circuit takes a collector current of the transistor Q2 and passes equal currents to the emitter circuit of Q1 and to an output load resistance R4. A diode connected transistor Q4 provides for the receiving of the input current and transistors Q3 and Q6 provide the output currents. The resistances R2, R3 and R5 are current equalizing elements and are selected to be equal to one another. An emitter resistor R1 is connected in the emitter circuit of the transistor Q2.
The output, V.sub.out, of this circuit has generally been described in the prior art by the equation: EQU V.sub.out =(R4/R5) (R2/R1) V.sub.in.
To increase the amplification of the circuit, the resistance R1 must be decreased toward zero. In practice, however, when this is done the amplification factor proves to be relatively low and non-linear distortion is not avoided. Moreover, the direct current bias point of the circuit will be indeterminate.
This is for two reasons. Firstly, the current mirror circuit has relatively low accuracy so that the collector currents of transistors Q1 and Q2 are not equal, and secondly because even where such collector currents are equalized, there is an inequality in the base-emitter voltages of the two transistors Q1 and Q2, being of different types, PNP and NPN. Moreover, these voltages have opposite dependence during variations caused by the amplified signal.